1. Field of the Invention
This invention relates to an electrostatic air sampling devices for high efficiency sampling of bioaerosols that can include airborne bacteria, viruses, fungi, spores, etc. and methods for using the devices.
2. Description of the Related Art
Numerous devices are available for collection of airborne microorganisms (bioaerosols) from outdoor and indoor environments. Some examples include impingers, impactors, centrifugal devices, cyclone devices, and forced-air electrostatic devices. Bioaerosols include microorganisms such as bacteria, viruses, fungi, fragments of microorganisms, air-borne pollens, toxins, etc. Bioaerosols can result from natural processes such as pollen releases by plants, or from human activities by inadvertent releases such as in food processing plants, poultry hatcheries, operating rooms, communicable diseases, etc., or intentional releases such as agricultural or battlefield releases. The concentration of specific bioaerosols of interest can be quite low and the challenge usually is to collect viable microorganisms in a large air sample.
One group of samplers are impingers which collect samples in liquid. The longer an impinger is operated the greater the loss of viable cells due to aeration. Collection directly into liquid provides some protection for microorganisms versus other methods and allows initiation of damage repair caused by the rigors of aerosolization, aerosol residence time, and collection. Aggregates of cells which would grow as a single colony collected by an impactor sampler, are broken up in the impinger liquid. This makes it possible, through appropriate titrations, to enumerate the total culturable cell per volume of air. The AGI-30 impinger is an all-glass unit which draws aerosols through an inlet tube curved to simulate the human nasal passage. A one-half atmosphere vacuum is drawn across the tube so that a choked-sonic flow condition is maintained with a typical flow rate of about 12.5 L/min. This flow rate has been found to be useful for collecting microbial particles in the respirable range of about 0.8 to 15 μM. The sample is directed into a collecting medium of water. The very low intake air flow rate can be a drawback especially if very low concentrations of particulates in the environment need to be sampled during a short time such as the typical 30-minute maximum operational time of the AGI-30. Some impingers have been developed to improve the viability of collected bioaerosols.
Another type of sampler is an impactor sampler which is useful for determining the size distribution of airborne particles. The sampler impacts particles onto agar surfaces using centrifugal forces. Collection by impaction makes it possible to enumerate the number of colony forming units (CFU) per unit volume of air. These evaluation methods rely on multiplication of microbial cells on nutrient media and therefore cannot detect microorganisms unable to grow because the media is inappropriate or the cells have been damaged by the stresses of aerosolization and/or collection. One example is the Andersen multi-stage cascade impactor which has a similar collection efficiency for microorganisms to that of the AGI-30 impinger. It provides relatively low intake air-flow rates of about 28.3 L/min. The Andersen six-stage and two-stage viable particle sizing impactors use vacuum pumps for operation which are noisy, heavy, and require an a.c. electrical source which limits usage in locations where electricity is not available or where size and weight must be minimized. Other examples of impactor samplers can be found in U.S. Pat. No. 6,514,721 (Spurrel, Feb. 4, 2003), U.S. Pat. No. 6,217,636 (McFarland, Apr. 17, 2001), and U.S. Pat. No. 6,101,886 (Brenizer et al., Aug. 15, 2000).
Some centrifugal samplers are lightweight and self-contained. In one model, airborne particles are impacted on an agar strip. Such samples can be useful for collecting samples in locations that are difficult to monitor with other methods such as inside duct work. Collection efficiency is a function of particle size and tends to be greater for larger sized particles.
Slit to agar samplers aspirate air by a vacuum through a slit and the airborne particles are impacted onto a rotating agar surface which allow differences in the aerial bioburden over time to be observed. This type of sampler may give lower counts versus impactors or impingers and may not be efficient for trapping small particles or useful in areas that have low numbers of culturable particles.
Cyclone samplers separate particles from a main flow and concentrate them in an adjacent recirculating chamber without impact. The main particle-laden flow follows a wall that curves away from the original flow direction. Although the wall forms the inner boundary of the main flow, its outer boundary is formed by an adjacent flow, often a confined recirculating flow, into which particles are transferred by centrifugal action. The particles are separated from the main flow by crossing a dividing streamline that separates the main flow stream from an adjacent flow stream. If a confined recirculating chamber geometry is used, particle concentrations in the recirculating region can be greatly increased relative to the main stream. Some of the drawbacks are turbulent mixing produced by shear-layer roll-up which can limit particle-concentration enhancement at high flow Reynolds numbers, and difficulty in sample removal from the recirculating chamber for analysis. Examples of this technology can be found in U.S. Pat. No. 6,156,212 and the Research Institute International flyer on the SASS 2000 Smart Air Sampler.
Filtration samplers are also used to collect culturable cells onto solid filters or soluble materials. Membrane filters have been found to recover similar numbers of fragile microorganisms to those recovered by agar surface impaction devices. Solid filter samplers may adversely influence the survival of airborne microorganisms due to impaction and dessication, and they recover fragile microorganisms with an efficiency similar to that of agar surface impaction devices. The use of gelatin air filters increases survival by decreasing the effect of dessication. Pore size must be carefully selected so those particles carrying bacteria, viruses, and fungi are retained on the filter.
Collection of culturable airborne particles through precipitation is achieved by samplers that collect air and electrostatically or thermally precipitate particles onto a thin flowing film of collecting fluid. Most are used for qualitative determination of pollen and fungal spores for use in allergenic evaluations. Examples of these type of collectors are the Rotorod, Burkard, LVS, and Samplair samplers. See also U.S. Pat. No. 5,855,652 and Allergy 2000, Volume 55, 1148–1154, 2000.
While various systems have been developed for sampling of airborne bacteria, viruses, fungi, spores, etc., there remains a need in the art for a more effective system for sampling viable and non-viable microorganisms and particles from bioaerosols using a device that causes minimal damage to viable organisms, has no moving parts, has a high collection efficiency, and is easily transported and disinfected. The present invention described below meets these needs and is different from the related art systems.